Method of working refractory metals



' 1.6 July 12,1927. F. KOREF ET AL 35 793 METHOD OF WORKING REFRACTORY METALS Filed April 29, 1924 Inventor-s: Ffinz KoreF; Kurt, Moers;

Their Abbornes.

1 '1 a macrocrystal wire being passed Patented July 12 1927.

UNITED STATES PATENT OFFICE.

FRITZ KORE! AND KURT MOERS, OF BERLIN-CHARLOTTENBURG, GERMANY, AS- SIGNORS TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

METHOD OF WORKING REFRACTORY METAL S.

Application flied April 29, 1924, Serial No. 709,878, and in Germany July 14, 1923.

The present invention relates to the working of refractory metals, such for example as tungsten, in the form. of large crystals known as macrocrystals, a single crystal, .or two or three crystals, constituting the entire part of the cross section of a wire or band, as distinguished from the minute crystals, known as microcrystals.

In macro-crystalline tungsten the crystals may be joined end to end, the crystal boundaries ending essentially across the wire, or the crystals may overlap each other so that the boundaries between the crystals extend essentially lengthwise of the wire. The

-macro-crystalline wire having boundar es extending across the wire has been designated herein as abutting crystal metal to distinguish this structure from overlapp ng crystal metal. Sections of wire consisting of a'single crystal will be referred to as unicrystal wire.

Wires or bands in the abutting crystalline state exhibit joints at separated points which are due to the junction of individual crystals, and which impair the usefulness of the wires or bands. The quality of wire consisting of overlapping crystalsdepends primarily on the completeness of the anchoring of the overlapping crystals to one another.

It is the object of our invention to 1mprove the physical properties of wires or other wrought articles of tungsten or other refractory metals which arecapable of being worked in the macrocrystalline condition.

The accompanying drawing shows in Fig. through a die; Fig. 2 illustrates an arrangement for progressively heating long wires and Figs. 3 and 4: respectively illustrate the crystal boundaries'before and after drawing.

A measure of the quality .of the abutting crystals or overlapping crystals is given by the easily established tensile strength. WVhile jointless pieces of unicrystal wires of tungsten atordinary temperature have a tensile strength of about 110 to 120 kilograms per square-millimeter, wires which have the defect of oints between abutting crystals,- show tensile strengths which vary greatly for different samples, and being as much as half as great in most cases. overlapping crystals may strength in the range of about 90 grams per square millimeter.

Wires consisting of to 120 kilo- Only when may be very have a tensile" than in the early drawing steps. The draw- 1 ing down, therefore, becomes progressively more difficult. It is notwithstan ing pos sible to draw down wire-shaped crystals to.

considerable degree. For example wireshaped tungsten crystals of a diameter of 0.1' millimeter may be drawn-down to 0.03 to 0.04 millimeter. The tensile strength increases in an increasing measure with the change in cross-sectionv and can increase up to about 180 kilograms per square millimeter. If the drawn crystal, wire is heated to the temperature of the lighting body of an incandescent lamp, then there takes place suddenly a basic variation in the crystal structure. A so-c'alled re-crystallization takes place, which consists in the formation of new crystals, the number and size of which different and the grain limits of which may vary irregularly. The wire has become brittle and its tensil strength has been reduced to values of about 30 kilograms per square millimeter. The new grain limits produced can be made also visible by etching. The crystal therefore has lost its most important characteristic by the deformation, namely the property ofremaining flexible after heating to bright incandescence. It has now been found that after but a few reduction steps the crystals can be heated to the highest incandescence without the occurrence of recrystallization or grain disintegration. The crystals again take ontheiroriginal properties in this case in an increasing degree with a protracted heating. The tensile strength',which, for example, may have been increased to 140 to 150 kilograms, again returns to its original value and it is possible by the choice of the temperature and the duration of the heating, to

Recrystallzation occurs with a subsequent heating only when the deformation has reached a limiting or maximum value, or has exceeded it. This limit and the beginning of the recrystallization connected therewith is different with different wire thicknesses. lVith very thick wires, say, of several millimeters. the limit may already be exceeded with a very slight reduction of crosssection,'

while with fine wires the reduction may be more than 50% of the diameter without reaching the limit. The deformation limit and the threshold of recrystallization varies further in accordance with the purity of the metals. Thus, for example, tungsten macrocrystalline wires which contain thorium oxide, can be reduced farther without a subsequent recrystallization than wires of pure tungsten. Furthermore, by the nature of deformation the reaching of the threshold of recrystallization is determined, namely it is also essential whether smaller or larger drawing steps are employed, or the drawing process is effected more or less quickly.

On the ground of these new and basic observations for the working of macrocrystals which were not to be foreseen, in any manner, it is possible to manufacture wires and bands, and the like, which far exceed in quality the known abutting or over-lapping crystal wires.

In accordance with our invention the anchoring of the adjoinin crystals can be improved at will without liarmfully affecting the other qualities of the wire by drawing or otherwise deforming the macrocrystalline crystal metal to an extent short of the critical value at which recrystallization will occur and then heating to a temperature sufficiently high to restore pliability.

Our invention is carried out in the following manner. It is started from a drawn wire or a squirted filament, of tungsten for in stance which is given in known manner by a thermal treatment a macro-crystalline structure. Such wire or filament with abutting or over-lapping crystals is then slightly reduced in its section by drawing, rolling, swaging, hammering, or the like. The recrystallization limit is determined by trial, for example, by heating a small sample for a short time to white heat, and then noting whether or not it has become brittle. Then the reduction in section has proceeded to a point short of the threshold of recrystallization, the reduced macro-crystalline wire or filament is very highly heated, if necessary, close to the melting point, whereby the softness which has been reduced by the deformation is restored. The heating can be carried out by passing an electric current through the wire.

An example of execution of the invention is shown inthe accompanying drawing. Fig. 1 shows diagrammatically a macromamas crystalline wire a which is reduced in section by passing through a die Z) in accordance with the usual wire drawing procedure. The reduced wire a then, as shown in Fig. 2, is c0ntinuousl y passed between mercury contacts 0 of an electric circuit. The mercury contacts 0 are located within a closed chamber f continuously supplied with hydrogen. The part of the wire a between the contacts 0 is by reason of the electric current heated to a temperature close to the melting point.

The deformation (Fig. 1) in conjunction with heating (Fig. 2) may be repeatedas often as may be desired. Thereby the contact surfaces (Z of the mutually overlapping crystals 6 which may have the shape shown. in Fig. 3 are drawn lengthwise as shown in Fig. 4, whereby the interlinking of the crystals 6 is very essentiall improved. Thus overlapping crystals are 0 tained whose tensile strength over their entire length is equal or approximately equal to the tensile strength of a jointless length of wire consisting of a single crystal. The hitherto unavoidable joints running transversely in abutting crystal wire, which are so harmful in use, are greatly lengthened in proportion to the cross section, and consequently the interlinking of the crystals is made more complete at these points, so that the danger of breakage is greatly decreased.

The use of the wires made in accordance with the invention thus represents an essential improvement in the manufacture of incandescent lamps. If desired, the last heating of the wire may be combined with the winding of the wire into helices.

The heating of the reduced crystal wire can also be effected without moving the same, for example it can be kept durin the heating between fixed points. The last eat-- ing of the wire can take place after mounting the wire in the lamps.

The following specific example may serve to explain the process. An overlapping crystal wire of tungsten, with an addition of 0.7 5% of thorium oxide, of a. diameter of 00? millimeters is reduced in two drawing steps to 0.06 millimeters, then by being drawn through two contacts, which are 2 centimeters apart, it is heated in a hydrogen atmosphere in a continuous manner to the highest white heat, the feeding speed being about 20 centimeters per minute. The wire then is drawn in two further steps to 0.05 millimeter, diameter, and again brought as before to the highest incandescence.

W hat we claim as new and desire to secure by Letters Patent of the United States, 1s:-

1. The method of treating single crystal refractory metal which consists in repeatedly and successively deforming said metal short of the threshold of recrystallization under conditions favorable to the interlinkage of macrocrystals, thereby incidentally hardening the metal and thereafter heating it to a temperature at which the hardness of said metal is reduced.

2. The method of treating macro-crystalline tungsten whichconsists in successively mechanically reducing the same in crosssection to a point no further than the threshold of recrystallization, and thereafter heating to a temperature sufiiciently high to decrease the hardness.

3. The method of treating wires of macrocrystalline tungsten which consists in alternately drawing said wires at a temperature below incandescence almost to the threshold of recrystallization and then heating to a temperature of bright incandescence to restore the original softness of said metal.

4; The method of increasing the tensile strength of wire consisting of macrocrystalline tungsten comprising overlapping crystals which consists in reducing the diameter of said wire mechanically without heating to a degree insufficient to cause recrystallization upon heating to incandescence, and then heating said wire to a. temperature at which the softness characteristic of said wire before working has been restored.

In witness whereof, we have hereunto set our hands this 7th day of April 1924. r

' FRITZ KOREF. KURT MOERS. 

